Muscle health and performance in monozygotic twins with 30 years of discordant exercise habits

Abstract

Several measured differences are the largest reported between MZ twins (TT expressed 55% more MHC I fibers, 12.4 ml/kg/min greater VOmax, and 8.6% lower body fat% vs. UT). These data collectively (a) support utilizing chronic endurance training to improve body composition and cardiovascular health and (b) suggest the cardiovascular and skeletal muscle systems exhibit greater plasticity than previously thought, further highlighting the importance of studying MZ twins with large (long-term) differences in exposomes.

Extraordinary Fast-Twitch Fiber Abundance in Elite Weightlifters

“…HG also greatly exaggerated MHC IIx, particularly in individuals with >4% MHC IIa/IIx. The inability of HG to account for MHC IIa/IIx explains why MHC IIx appear common in some studies (48) even though their actual abundance in healthy human skeletal muscle is extraordinarily rare; typically <0.1% (9, 22–25, 27) and 0 of the >2,100 isolated fibers from the current sample. Thus, the seeming conversion of MHC IIx to IIa with exercise is more precisely IIa/IIx changing to IIa.…”

“…For these reasons MHC IIa/IIx are typically inversely associated with muscle health and physical activity (9, 43). Yet, the heavyweights (male and female) expressed irregularly high concentrations (24%) and accounted for 91% of all MHC IIa/IIx myofibers.…”

“…Following 30 minutes of supine rest, athletes underwent a mid-muscle belly (approximately halfway between the greater trochanter and patella) biopsy of the vastus lateralis . A detailed description of the biopsy procedure has been previously described by our lab (9, 22, 23, 33). Briefly, a small area of the thigh was numbed by injection of a local anesthetic (Xylocaine/Lidocaine without epinephrine).…”

“…Thus, SF indicates how frequently each isoform exists but cannot address how much area each FT occupies within the muscle. HG addresses the latter, but cannot delineate hybrids, therefore inaccurately quantifying FT% (9, 21-25, 27).…”

“…Extensive evidence indicates endurance athletes possess a slow-twitch myofiber majority (9, 10, 12, 14, 15), yet relatively few investigations have explored FT in speed, power, or strength athletes. Initial research in the 1970-80’s found resistance-trained men expressed high quantities (~60-65%) of fast-twitch fibers (11, 12, 15, 16), which was substantiated by later studies on elite powerlifters (17) and national-caliber ( Olympic ) weightlifters (8).…”